This invention is directed to a single package zinc-filled coating which when applied to a metal surface imparts galvanic protection. More specifically, this invention is directed to stable zinc-rich coatings comprising mixtures of partially hydrolyzed ethyl silicate and zinc dust in an organic solvent with an inorganic catalyst. This composition is stable at ambient temperatures but is highly reactive in atmospheric moisture and therefore cures rapidly when exposed to air. Zinc metal-containing coatings utilizing hydrolyzed ethyl silicate as the binders are well known compositions which have been used for the protection of ferrous metal surfaces. In accordance with this invention, single package corrosion resistant coatings are obtained from zinc in a mixture of hydrolyzed ethyl silicate in an organic solvent with an alkali metal salt catalyst. These mixtures are substantially stable in the absence of moisture and exhibit little or no viscosity increase or gasing during storage and can be applied by conventional methods to obtain a completely cured film under ambient conditions. These coatings exhibit excellent corrosion resistance at thicknesses as low as 0.5 mil and therefore are particlarly useful as primers on various metal surfaces such as steel etc.
Zinc-filled protective coatings have been used as corrosion inhibiting compositions as disclosed, for example, in U.S. Pat. Nos. 3,056,684 and 3,615,730. These zinc-filled coatings, however, were obtained by adding zinc to the vehicle at the time of packaging and thereby the reactivity of the zinc with the vehicle caused gelling thereby resulting in an unacceptable short shelf life. Moreover, many of the single package zinc-filled coatings have the disadvantage of evolving gas i.e. hydrogen in the container when stored at ambient or room temperatures for periods exceeding 60 days or more. Thus, it was not possible to prepare a single package zinc-filled silicate coating which is stable for periods of several months or more and which can be used for coatings having the desired hardness, shelf life, pot life, and film forming qualities.
It is known that ethyl silicate reacts with moisture and hydrolyzes to silica and therefore it has been proposed that the silicates be used as a vehicle for paints. However, in the absence of a catalyst, the silicates react slowly and therefore are not practical for use as coatings unless the silicate has been hydrolyzed to a fairly stable solution of a partially hydrolyzed silicate. The coating will absorb even relatively small amounts of atmospheric moisture thereby completing the hydrolysis of the silicate to a fully hydrolyzed silicate. There is however a practical limitation as to how much this conversion can be accelerated by prehydrolyzing the silicate. It has been found that partially hydrolyzed ethyl silicate, for example, can be used to make a good zinc-rich paint to protect steel and other metals from corrosion. This is accomplished when the partially hydrolyzed silicate e.g. hydrolyzed to levels of 40-65% is mixed with metal dust such as zinc dust and applied to the metal substrate as a coating. However, in order to obtain optimum conditions to harden zinc-rich paints, there is a minimum requirement of atmospheric conditions i.e. humidity and temperature.